The basics of dust-explosion protection - STAHL - #8

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Thus it is impossible to provide generally valid characteristics for a particular kind of dust. There are wide variations for identical dusts. Depending on conditions, limit values often cannot be determined exactly; nor then, can the risk of ex­plosion. The minimum ignition energy can also vary to a large extent. In Table 3, the limit values for several products are listed; that is, values that border on the hazardous. In rare cases, the dust/air mixture in question can also be ignited at considerably lower energies.

Note that it is not possible to directly infer the minimum ignition temperature from the minimum ignition energy, and vice versa.

Table 2 includes detailed commentary on the characteristics for dusts.

Table 3: Examples of Explosion Characteristics for Dusts

Functional Description of a Modified Hartmann Tube (Figure 5): Not every spark is capable of ignition. The deciding factor is whether the energy added to the mixture is sufficient to initiate self-sustaining combustion of the entire mixture. The modified Hartmann tube is used as a qualitative test apparatus for the minimum ignition energy. At the base of the tube is an atomising cone which is employed to swirl a defined quantity of the dust being investigated. A compressed air blast of 7 bar swirls the dust in the glass cylinder and the resulting mixture is then ignited by a spark created between two electrodes.

A test is considered to be positive if the indication instrument shows a deflection of the hinged cover or, if a dust fire occurs (even if the hinged cover is not moved).

Figure 4: Determining a Dust's Minimum Ignition Energy

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